1. Field of the Invention
The present invention relates to a scan converter for converting R, G, and B signals output from a computer into a video signal from which a television receiver can reproduce and display images, and particularly to a scan converter for converting video signals output from a personal computer into a video signal from which a television receiver can reproduce and display images.
2. Description of the Prior Art
Usually, video images output from a personal computer are displayed on a dedicated monitor by feeding R, G, and B signals and horizontal and vertical synchronizing signals from the personal computer directly to the monitor. However, it is also possible to display such images on a television receiver if the images output from the personal computer are converted into a television signal by the use of a scan converter before they are fed to the television receiver.
A television signal refers to, for example, a video signal conforming to the NTSC (National Television System Committee) standards, i.e. a signal that carries images as successive frames each composed of 525 scanning lines that are scanned in an interlaced way such that one frame is formed by two consecutive fields. The television signal also contains horizontal and vertical synchronizing signals having frequencies of 15.734 kHz and 59.94 kHz respectively.
Here, it is assumed that the frequency of the vertical synchronizing signal output from the personal computer is 59.94 Hz, i.e. equal to the frequency of the vertical synchronizing signal of the television signal. Moreover, it is assumed that the frequency of the horizontal synchronizing signal output from the personal computer is 31.468 kHz, i.e. twice the frequency 15.734 kHz of the horizontal synchronizing signal of the television signal. This is because the personal computer is configured to output signals that form images in an non-interlaced way.
The R, G, and B signals output from the personal computer are individually converted into digital signals by an analog-to-digital converter (A/D converter). These digital signals are temporarily stored in a line buffer. The digital signals stored in the line buffer are individually converted into analog signals by a digital-to-analog converter (D/A converter), and are then fed to an encoder for analog signal processing.
The vertical synchronizing signal output from the personal computer is fed directly to the encoder. On the other hand, the horizontal synchronizing signal output from the personal computer is fed to the encoder through a frequency divider that converts the frequency of the signal to half its original frequency. Thus, when the horizontal synchronizing signal reaches the encoder, it has a frequency of 15.734 kHz, i.e. the frequency of the horizontal synchronizing signal in the NTSC television system. The frequency divider performs frequency division by outputting a pulse every second time it receives a pulse.
The encoder first reads in every second scanning line from the analog R, G, and B signals, and converts it into a luminance signal and a color difference signal. The encoder then adds to the luminance signal the horizontal and vertical synchronizing signals that it receives separately. Thereafter, the encoder modulates the color difference signal with a chrominance subcarrier, and adds the resulting signal as well as color bursts to the luminance signal. In this way, the encoder converts the R, G, and B signals into a composite video signal, and outputs it as a television signal. The television signal is fed to a television receiver to display images thereon.
However, the conventional scan converter requires an analog-to-digital converter. Moreover, since its encoder processes analog signals, the conventional scan converter requires an oscillator for generating a chrominance subcarrier. These factors have been preventing simplification of the construction of conventional scan converters and thus reduction of their cost.